This invention relates to a novel method with materials and devices for the suppression of electromagnetic interference, particularly within the radio frequency range. More specifically the invention relates to the material of construction and design of brush holder assemblies used with commutating brushes for electric motors, generators and other rotating electrical equipment such as rotary switches.
Commutating brushes are used to maintain electrical contact between a stationary member (or stator) and a rotating member (or rotor) of rotating electrical equipment such as electric motors, generators, rotary switches, and the like. In most such equipment there is a normal current reversal and electrical connection dynamics involved in the operation of commutating devices which results in the generation of electromagnetic interference (EMI). This EMI may be conducted and/or radiated to neighboring machines and devices and result in interfering with the operation of such neighboring machines and devices thus causing serious reliability or performance problems in complex electrical systems such as those utilized in automobiles, airplanes, etc. Of particular concern are EMI signals lying within the radio frequency range referred to as radio frequency interference signals (R.F.I.), (including not only the broadcast AM and FM portions of the frequency spectrum, but extending up to and including the microwave regions. Previously, EMI/RFI has been suppressed using various add-on combinations of electrical components which are directly added to the electrical system or systems which might be affected by EMI/RFI. Capacitors and radio frequency choke coils are added in various configurations to contain EMI/RFI near its point of origin or at distant points to filter the radio signals at the point of reception. For example, U.S. Pat. No. 3,189,824 discloses the use of RFI suppression circuitry at the reception end in order to improve the performance of a radio receiver. U.S. Pat. No. 4,845,393 discloses the use of an EMI/RFI suppression circuit in the construction of an integrally built fuel pump end cap for reduction of EMI/RFI emissions from or by the fuel pump.
The present invention provides a novel means for suppressing EMI/RFI at its point of origin by utilizing EMI/RFI suppressing materials in the construction of a brush holder assembly on an electric machine such as an electric motor, electric generator rotary electric switch, and the like. The EMI/RFI suppression is achieved as a result of the material composition of the brush holder assembly. If desired, the EMI/RFI suppression may be further enhanced by the use of add-on electrical components such as capacitors or radio frequency choke coils. However, the invention does not require the use of such additional circuit components to suppress EMI/RFI. It also provides for the integration of commutating brushes contracts, springs, shunt wires and add-on EMI/RFI in addition to that achieved through the materials used in fabricating commutating brush holder assemblies and like switching devices with considerable design flexibility and wide ranging applications..